1. FIELD OF THE INVENTION
The present invention relates to a spindle motor suitably used for rotatively driving a hard disk as in a magnetic disk unit.
2. DESCRIPTION OF THE RELATED ART
There has been hitherto a spindle motor to rotatively drive a hard disk serving as a magnetic recording medium as in a magnetic disk unit. In such a spindle motor, a rotary shaft is fitted into a boss fixing the hard disk. The boss and the rotary shaft are rotated together. (Refer to Japanese Utility Model Laid-Open No. 1-61858.)
As shown in FIG. 1, in a spindle motor 1, a cylinder-shaped bearing-retaining portion 2A is formed at the center of a flange-shaped bracket 2. A rotary shaft 6 is rotatably supported through bearings 3 and 4 by the internal peripheral surface of the bearing-retaining portion 2A.
One end of the rotary shaft 6 is fitted into a fixing hole 8A of a boss 8. The rotary shaft 6 and the boss 8 are constructed so as to rotate together.
A hard disk 11 is secured by a fixing member 12 to an external peripheral surface 8B of the boss 8.
A coil portion 16 composed of a winding wound around an iron core 15 is secured to an external periphery of the bearing-retaining portion 2A. A rotary yoke 17 and an annular magnet 18 are also secured to an internal peripheral surface 8C of the boss 8 so as to face the coil portion 16.
Thus the introduction of an operating electric current to the winding 14 causes the rotary shaft 6 and the boss 8 to rotate together, thereby rotatively driving the hard disk 11.
By making a magnetic disk unit smaller and thinner, it is possible for users to use the magnetic disk unit more freely when it is mounted in, for example, a small portable computer.
There is thus a demand that the spindle motor for rotating such a magnetic disk be made even thinner. For example, the entire spindle motor 1 shown in FIG. 1 can be made thinner if the fixing hole 8A of the boss 8 (that is, the thickness of a portion 19 where the boss 8 is joined to the rotary shaft 6) can be made shallower. (The portion 19 is hereinafter referred to as a joining portion 19.)
In the spindle motor 1 as constructed above, a position where the hard disk 11, secured to the external peripheral surface 8B of the boss 8, is attached is offset with respect to the joining portion 19 of the rotary shaft 6 and the boss 8 in a direction in which the rotary shaft 6 rotates (hereinafter called simply a rotary shaft direction) For this reason, as shown in FIG. 2, when the hard disk 11 is secured to the boss 8, and impact force F.sub.0 is applied to the hard disk 11 from, for instance, a direction indicated by an arrow "a" (or radial direction) moment force is applied to the joining portion 19 in a direction indicated by an arrow "b". Because of the application of the moment force, when the rotary shaft 6 is made of, for example, hard stainless steel, and the boss 8 of a soft aluminum material, the fixing hole 8A in the boss 8 is deformed in the joining portion 19.
In other words, when the impact force F.sub.0 is applied to the hard disk 11 from, for example, the radial direction, force F.sub.1 acts on the center of gravity G.sub.1 of the joining portion 19 (of a rotary portion 1A) in the rotary shaft direction. The rotary portion 1A is composed of the boss 8, the hard disk 11, the fixing member 12, the rotary yoke 17, and the annular magnet 18. The force F.sub.1 is expressed by the following equation: EQU F.sub.1 =M X F.sub.0 ( 1)
where M is the mass of the rotary portion 1A.
When the force F.sub.1 acts, an offset yield stress acting on a unit area of a surface where the boss 8 is in contact with the rotary shaft 6 (hereinafter simply called a contact surface) is expressed as F.sub.2 in the following equation. The fulcrum of the boss 8 with respect to a direction b in which moment force is applied is given as O. When the following conditional equation (2) is satisfied, the fixing hole 8A or the contact surface of the boss 8 is diagonally and plastically deformed: ##EQU1## where z is a unit area of the contact surface, points O and P being the upper and lower ends of the joining portion 19.
Thus, the hard disk 11 secured to the boss 8 will not be positioned precisely at right angles with the rotary shaft 6, as indicated by dash-and-dot lines 1B in FIG. 2. When the spindle motor 1 is rotated to rotatively drive the hard disk 11, a surface 11A of the hard disk 11 onto which information is recorded (hereinafter referred to as an information recording surface 11A) is appreciably vibrated up and down, and what is called mechanical vibration characteristics are deteriorated. As a result, the information recording surface 11A comes into contact with an information reading head (not shown), thereby damaging the information reading head, or making it difficult to accurately read information. The information reading head is disposed about 0.2 .mu.m above the information recording surface 11A.